Method of charging aluminium furnaces



Oct. 31, 1961 M. C'JVROM SEM METHOD OF CHARGING ALUMINIUM FURNACES Filed Dec. 18, 1958 INVENTOR. MATHIAS OVROM SEM A TTORNEYS.

United States Patent Qfice 3,006,825 Patented Oct. 31, 1961 3,006,825 lVIETHOD F CHARGING ALUMINIUM FURNACES Mathias Ovrom Sem, Smestad, Oslo, Norway, assigncr to Electrokemisk A/S, Oslo, Norway, a corporation of Norway Filed Dec. 18, 1958, Ser. No. 781,4tl Claims priority, application Norway Dec. 19, 1957 3 Claims. (Cl. 204-67) The present invention has for its object a method for charging furnaces for melt-electrolytic production of aluminium, especially such furnaces where the current is supplied to the anode by means of vertical contact rods which at the same time serve as suspension of the anode. Such furnaces are usually provided with an arrangement for collection of the furnace gases in concentrated for-m. This arrangement usually consists of a ring surrounding the lower part of the anode near the bath surface.

As the electrolysis proceeds, the content of A1 0 in the bath will be reduced and when a certain limit has been reached, usually about 1.5%, the bath tension will rise. This is usually indicated by a bell which sounds or a lamp which lights up. The phenomenon is called anodic effect or lighting of the furnace. Wh n the anodic effect takes place new A1 0 must be supplied to the furnace. This is usually done by breaking down the crust at one or more places at the long sides of the furnace and then introducing A1 0 through these openings. When the crust is broken down in this way, some of the furnace gas will of course escape through the openings and get lost. This gas is rich in both CO and fluorine compounds, and in a large furnace hall where many furnaces operate in series considerable values will get lost both as COgas and as valuable fluorine compounds. Furthermore the fluorine gases are detrimental to the neighbouring vegetation.

It is therefore desirable to be able to avoid the frequent breaking down of the crust, for example by even and continuous charging of A1 0 Several methods have been developed for such charging, but most of them have met with the difllculty that the oxide is so moist that it will easily stick in hoppers and feeding pipes.

According to the usual methods the furnace gas is combusted in a burner in connection with the furnace as shown for example in Jouannet Patent 2,526,875, and then led to a washing device where its content of fluorine can be recotered in the form of cryolite or the like. The cryolite is then retur ed to the furnace. Hereby the fluorine content is recovered, but the heat of combustion of the CO-gas is lost.

The inventor has found that this heat of combustion generated in the burner can be utilised by directly after combustion leading the gases in counter current to A1 0 which is supplied to the furnace through a hopper which is connected with a feeding device for continuous feeding. Hereby the oxide is dried and the above described drawback of sticking in the feeding apparatus is avoided and at the same time a certain preheating of the oxide is attained. Warm oxide is also more easily soluble in cryolite than cold oxide.

The gas can if desired, be led through a dry cyclone located before the burner. Hereby dust and condensed tar particles will be separated out. Such a cyclone has a very favourable effect as the dust will agglomerate on account of the tar so that the dust particles will ball together and increase in size.

According to the present invention the content of F and HP in the gases will also become adsorbed to the surface of the aluminium oxide and at the same time fluorine-containing dust still present will settle. The gas is led through the feeding device and/or the hopper in such a way that at the same time it will transport the oxide forwards towards the furnace. This is attained by leading the gas into the hopper through slots or holes in the inclined side walls of the hopper. If desired one can also use perforated or porous plates in these walls. The gas will then blow through the charge and dry it. The dried oxide will partly become fluidised in the gas current and on account of whirling motion created thereby an intimate contact between gas and charge is obtained which facilitates the adsorption of the fluorine compounds. The gas current can also be used for transport of the oxide from the main storage bin to the hoppers of the individual furnaces. The hoppers should then be so connected that each of them will have an overflow device. When one hopper is full, the excess of oxide will be transported to the next one, etc. in this way all hoppers can always be kept full.

Dry and preheated A1 0 is as stated above more easily soluble in the cryol-ite melt than moist unheated oxide and has less tendency to form crusts. The danger of the furnace clogging up below the charging device is thereby reduced and a simple mechanical device which presses the charge into the bath will be suflicient. The above described way of adsorption of fluorine makes the complicated arrangements for recovery of fluorine superiluous as the precipitated fluorine will automatically be returned to the furnace. The fluorine losses are very much reduced and the economy of the process improved.

The invention is schematically illustrated in the appended drawing which shows a section through a feeding hopper placed on the long side of a furnace close to the anode. The gas current is indicated by arrows. In the drawing 1 is the hopper filled with aluminium oxide 2. The combusted furnace gas is led from the burner (not shown on the drawing) through the pipe 3 which is provided with a control valve 4. The hopper 1 has a double bottom and the inner bottom 5 is provided with slots or openings through which the furnace gas can pass. This bottom can also be made of perforated plate. 6 is a feed pipe leading from the hopper to the smelting chamber. The pipe can if desired be provided with a device for controlling the supply of charge. In the pipe 6 the charge flow in counter current to the gases escaping through the charging hole in the crust 8. This hole is all the time kept open by means of the stoker 7, which at the same time presses the oxide down into the bath 9. 10 is aluminium oxide sealing between the crust and the charging pipe.

To assure a good distribution of gas through the bottom of the hopper it is advantageous to arrange a connecting pipe 11 between the gas pipe 3 and the feed pipe 6. This pipe should also be provided with a control valve 12.

The oxide is supplied to the hopper 1 from the hopper of the preceding furnace or from the main storage bin through the pipe 13. As mentioned above, furnace gas is used for the transport. it is, however, also possible to employ air pr ssure for such transport. When the hopper 1 is full, excess oxide is led through the pipe 14 to the hopper of the next furnace.

The drawing is only meant as an example of the applications of the present invention, which is not limited to During the experiments which have been carried out with the process according to the invention, we have found that more than 90% of the fluorides of the gases will be carried back to the furnace with the A1 0 to be charged, while only 10% of the sulphur of the gases is returned in the corresponding way. The adsorption is thus strongly selective.

The selective adsorption is of course independent of the charging of the furnace being continuous or batchwise. If the furnace is charged in the usual batchwise way by breaking down the crust when anodic efiect occurs, the furnace gas may simply be led through a container located outside the furnace which contains the alumina to be charged.

1 claim:

I. The method of charging electrolytic aluminum furnaces with alumina which comprises moving such alumina with the aid of gravity over a fluidizing bed toward the furnace, collecting gases from such a furnace, burning the combustible elements of such gases, passing the 20 hot gases from such combustion through the alumina on such fluidizing bed whereby such alumina is rendered more fluid and its flow toward the furnace is assisted and at the same time fluorides of the hot gases are absorbed by the alumina, and introducing such alumina into the furnace whereby charging of the furnace is aided and fluorides are returned to the furnace.

2. The method as specified in claim 1 in which the alumina is stored in a hopper having perforated, sloping bottom walls and the hot gases are caused to pass through such perforations whereby the alumina is fluidized by such gases and thereby its movement towards the furnace is assisted.

3. A method as specified in claim 1 in which the fluidized alumina after moving toward the furnace is charged into the furnace by mechanical means.

References Cited in the tile of this patent UNITED STATES PATENTS 2,593,741 Ferrand Apr. 22, 1952 2,713,024 Mantovanello July 12, 1955 2,859,160 Helling Nov. 4, 1958 OTHER REFERENCES Perry: Chemical Engineers Handbook, 3rd ed., 1950, page 835. 

1. THE METHOD OF CHARGING ELECTROLYTIC ALUMINUM FURNACES WITH ALUMINA WHICH COMPRISES MOVING SUCH ALUMINA WITH THE AID OF GRAVITY OVER A FLUIDIZING BED TOWARD THE FURNACE, COLLECTING GASES FROM SUCH A FURNACE, BURNING THE COMBUSTIBLE ELEMENTS OF SUCH GASES, PASSING THE HOT GASES FROM SUCH COMBUSTION THROUGH THE ALUMINA ON SUCH FLUIDIZING BED WHEREBY SUCH ALUMINA IS RENDERED MORE FLUID AND ITS FLOW TOWARD THE FURNACE IS ASSISTED AND AT THE SAME TIME FLUORIDES OF THE HOT GASES ARE ABSORBED BY THE ALUMINA, AND INTRODUCING SUCH ALUMINA INTO THE FURNACE WHEREBY CHARGING OF THE FURNACE IS AIDED AND FLUORIDES ARE RETURNED TO THE FURNACE. 